Strengthening our science education in the United States is essential to the future success of our country in the global marketplace. Immersing our elementary students with research-based quality science instruction is a critical component to build a strong foundation and motivate our students to become interested in science. The research for this study pertained to the type of elementary science instruction in correlation to academic achievement and gender. Through this study, the researcher answered the following questions: 1. What is the difference in achievement for elementary students who have been taught using one of the three science instructional approaches analyzed in this study: traditional science instruction, inquiry-based science instruction with little or no professional development and inquiry-based science instruction with high-quality professional development? 2. What is the difference in student achievement between inquiry-based instruction and non-inquiry based (traditional) instruction? 3. What is the difference in student achievement between inquiry with high quality professional development and inquiry with little or no professional development? 4. Do the three instructional approaches have differentiated effects across gender? The student achievement was measured using the 2010 fourth grade Pennsylvania System of School Assessment (PSSA) in Science. Data was collected from 15 elementary schools forming three main groupings of similar schools based on the results from the 2009 third grade PSSA in Mathematics and student and community demographics. In addition, five sub-group triads were formed to further analyze the data and each sub-group was composed of schools with matching demographic data. Each triad contained a school using a traditional approach to teaching science, a school utilizing an inquiry science approach with little or no professional development, and a school incorporating inquiry science instruction with high quality professional development. The five schools which provided its students with inquiry science and high quality professional development were Science Its Elementary (SIE) schools, as provided through a grant from the Pennsylvania Department of Education (PDE). The findings of the study indicated that there is evidence to suggest that elementary science achievement improves significantly when teachers have utilized inquiry instruction after receiving high-quality professional development. Specifically, the analysis of the whole group and the majority of the triad sub-groupings did result in a consistent trend to support science instruction utilizing inquiry with high-quality professional development compared to a traditional approach and an inquiry-based approach with little or no professional development. The gender analysis of this study focused on whether or not girls at the elementary school level would perform better than boys depending upon method of science instruction. The study revealed no relationship between approach to teaching science and achievement level based on gender. The whole group results and sub-group triads produced no significant findings for this part of the data analysis. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]